1. Field of the Invention
The present invention relates to a method for applying a printing ink composition to circuit boards, and more particularly, to a method of applying printing ink composition exhibiting improved printing properties, adhesion, flexibility and heat resistance.
2. Description of Related Art
Heretofore, cover films have been used as coating materials for printed circuits. This technique is that an adhesive is coated on one surface of a polyimide or polyester film and holes are made only at portions corresponding to connection terminals by means of punching or the like and alignment of position is effected on the circuit board by handwork and then they are adhered at an elevated temperature under a high pressure by a hot plate press.
This method can give good flexibility and is excellent from the standpoint of circuit protection, but there are some problems. For example, when a lead wire is attached to a terminal of a circuit board, it is necessary to punch the cover film previously to make a small hole. Therefore, when the circuit is communicated, the position alignment with the circuit board is difficult. In addition, the investment in a hot plate press is expensive. Further, the use of an adhesive is liable to cause smearing upon munching processing and the cost becomes high and there is a fear that adhesive will ooze out.
Even if physical properties of a film itself are good, there is the possibility that the use of an adhesive will lower said physical properties.
Furthermore, the functions themselves of the adhesives still have problems, that is, an adhesive capable of adhering sufficiently to both a cover film such as polyimide film and the surface of a printed circuit and exhibiting good heat resistance and electric characteristics has not yet been found.
On the contrary, a method of forming an insulating protecting film on a printed circuit board by a printing process (cover-lay ink) has been developed. However, cover-lay ink agents used at present are based on solder resists which have been heretofore used for rigid substrates, and therefore, flexibility is less and heat resistance and electric characteristics are poor.
Japanese Patent Application Laid-open No. 145717/1980 discloses a composition composed of epoxy acrylate resin and melamine resin, but this composition has low flexibility.
Further, Japanese Patent Publication Nos. 1395/1975 and 10636/1978 propose acrylic resin compositions such as sulfomethylene acrylates, phosphoric acid ethylene acrylates and the like, but these compositions have relatively good flexibility while the heat resistance is very poor.
Heretofore, in order to use polyimide and polyamide-acid solutions as an ink for screen printing, thixotropy agents have been used to impart thixotropy thereto.
However, the conventional thixotropy agents can not sufficiently exhibit the functions due to polarity of polyamide-acids and that of solvents used for dissolving the resin, and a large amount of a thixotropy agent is used so as to exhibit sufficiently the thixotropy.
Therefore, the addition of a thixotropy agent often causes the degradation of the physical properties as a film. In addition, the adhesion of the film to the substrate is disadvantageously lowered by adding a thixotropy agent.
For example, Japanese Patent Publication No. 37073/1987 and Japanese Patent Application Laid-open Nos. 221172/1988 and 121364/1989 disclose that fine powder of silica is used as a thixotropy agent, but due to the polarity of solvents and resins, a large amount of silica powder should be used and thereby the flexibility and adhesion to a substrate are poor,
Japanese Patent Application Laid-open Nos. 179242/1982, 189260/1983, and 108068/1984 disclose that polyimide powder is used as a thixotropy agent, but it is required to add polyimide powder in a large amount and thereby there are drawbacks that the resulting flexibility is poor, the surface smoothness is inferior and shrinkage upon curing is large.
The present inventors have made researches so as to find a cover-lay ink resin composition for coating which can satisfy requirements for the various functions. As a result, it has been found that a composition comprising a partially imidized polyamide-acid solution as the main component can give excellent heat resistance, flexibility, electric properties and printing properties. In particular, it has been found that the flexibility can be improved to a great extent since a thixotropy agent in the form of a solid powder is not added.